The present invention relates to a device for detecting a properly focused state by utilizing the fact that the contrast of the image of an object to be photographed is a maximum when the lens is in focus.
It is a property of an optical image of an object formed by a lens that its contrast, that is, the difference between lighted and shaded areas of the image, is a maximum when the lens is in focus. This phenomenon is attributed to the fact that the optical intensities (power spectrum) of spatial frequencies of an object to be photographed are a maximum when the lens is in focus. Therefore, if the maximum contrast position can be detected, according to this phenomenon, then the correct focus can be detected. A variety of automatic focus control devices utilizing this phenomenon have been proposed in the art.
Recently, a technique has been proposed in which a self-scanning type photoelectric element is provided for detecting contrast data. With this technique, a self-scanning type photoelectric element array made up of a plurality of microphotoelectric elements and a scanning circuit are arranged in an optical image forming plane and the optical image is electrically scanned to extract contrast data of the optical image.
A variety of techniques for electrically processing the output signal of the self-scanning type photoelectric element array so as to extract contrast data therefrom have also been proposed in the art. In Japanese Patent Application Nos. 107354/1978 and 164960/1978 corresponding to U.S. Patent application Ser. No. 102,071, filed Dec. 10, 1979 by the present inventor and which are here not mentioned as prior art but only to aid in a full understanding of the present invention, there is disclosed a device in which the output of each of the microphotoelectric elements forming the self-scanning type photoelectric element array is stored for one cycle time and the difference between the output thus stored and the output of a microphotoelectric element adjacent to or near the first minute photoelectric element is formed thereby providing a signal representing the contrast of the optical image. This device is advantageous compared to a device utilizing a conventional method in which a differentiation circuit is used to compute the differentiation value of the optical image. The detection method utilized by the device of the aforementioned patent applications will be referred to as "a difference output detecting method" hereinafter.
According to the difference output detecting method, as the contrast of an optical image decreases, the absolute value of the difference output also decreases. Automatic focus detection can be implemented by utilization of this variation. However, it should be noted that, as the average illuminance of the optical image varies, the difference output also varies. On the other hand, if the scanning speed of the self-scanning type photoelectric element array is maintained constant, then it is sometimes impossible to detect the difference output because the output of the element becomes saturated or is so low as to be masked by noise since the dynamic range of photoelectric output with respect to the quantity of light of such elements is typically not very wide.
In order to solve both of these two problems, a technique has been proposed in the art in which the scanning speed of the self-scanning type photoelectric element array is changed according to the average illuminance of the optical image applied to the element. That is, as the average illuminance of the optical image increases, the scanning speed is increased and, as the average illuminance descreases, the scanning speed is decreased so that the dynamic range of the device is increased and variations of the difference output due to variations of the average illuminance are prevented. In order to detect the average illuminance, a single photoelectric conversion element (hereinafter referred to as "an average illuminance detecting element" and the corresponding method will be referred to as "an illuminance compensation method" when applicable) is arranged near the self-scanning type photoelectric element array.
With the illuminance compensation method using the average illuminance detecting element applied to the above-described difference output detecting technique, a difference output independent of variations of the average illuminance can be obtained if the optical image is at rest. However, the difference output is changed if the optical image is moved perpendicularly to the optical axis. An ordinary camera is often held by hand and most of the objects photographed are in motion. In addition, the range of brightness of an object to be photographed is usually relatively wide. Accordingly, application of a focus detecting device using the self-scanning type photoelectric element array to an ordinary camera is disadvantageous in that the output varies even if the above-described illuminance compensation method is employed.
Accordingly, an object of the invention is to provide a focus detecting device in which all of the above-described difficulties have been eliminated and, although the specific feature of the technique for extracting the difference output of two microphotoelectric elements using a self-scanning type photoelectric element array is utilized, output variations due to variations in brightness of an object to be photographed are eliminated and no output variations are caused by shaking the camera or by movement of the object.
Another object of the invention is to provide a focus detecting device in which the dynamic range of photoelectric element characteristics with respect to the quantity of light is increased so that a contrast signal of sufficiently high amplitude can be obtained not only from objects of low brightness but also from objects of very high brightness.